LOAF

LOAF (List Of All Friends) is an extension to email that lets you send out address book data without compromising your privacy. It appends a hash-like data structure to each outgoing email, and collects similar attachments from the people who write to you. These files can be queried to see if they contain a given email address, but they can't be reverse-engineered to reveal the list of addresses used to construct them. LOAF lets you check whether someone emailing you for the first time is a complete stranger, or appears in the address books of some of your trusted correspondents. And as a decentralized application, it offers an interesting alternative to current social networking sites like Orkut or Friendster.